1. Field of the Invention
The present invention relates to a method of producing a polishing pad-use polyurethane foam and a polyurethane foam produced by such a method.
2. Description of the Related Art
With respect to a polishing method for smoothing a wiring formation face on a semiconductor substrate efficiently with high precision, a CMP (Chemical Mechanical Polishing) method has been adopted. This method is a polishing method in which chemical functions and mechanical functions are utilized in a combined manner, and makes it possible to flatten the semiconductor substrate (wafer surface) over a wide range.
Some of polishing pads to be used in this CMP method are formed by a polyurethane foam so that fine recessed sections are formed on the surface with polishing slurry being maintained therein.
With respect to a known technique for a producing method of such a polishing pad-use polyurethane foam, for example, Japanese Patent Application Laid-Open No. 11-322877 has disclosed a producing method. In this producing method, upon mixing and stirring two solutions of an isocyanate-terminated prepolymer and an active hydrogen-containing compound, unfoamed heat-expansive fine hollow spherical particles (hollow balloons) are preliminarily added and mixed in either of the prepolymer and the active hydrogen-containing compound, and by utilizing reaction heat released when the two solutions are allowed to react with each other and cured or externally-applied heat, the unfoamed heat-expansive fine hollow spherical particles are finely foamed in the urethane molded product so that the molded product is allowed to contain the heat-expanded fine hollow spherical particles.
Moreover, Japanese National Publication of International Patent Application No. 8-500622 has disclosed a producing method as another known technique. In this producing method, an isocyanate-terminated prepolymer and an active hydrogen-containing compound (MBOCA) together with expanded fine hollow particles are simultaneously put into a special mixer in which the fine hollow particles are fully kneaded in a short reaction time, and evenly dispersed and mixed.
However, the hollow balloons of this expanded type are a powder material having a specific gravity of about 0.04 g/cm3, and it is difficult to fully knead and evenly mix the hollow balloons in this state with the urethane solution. The reason for this is explained as follows: since there is a great difference in density between the reaction solution and the hollow balloons, the hollow balloons tend to float up during the curing process, causing a difficulty in forming an even product. With respect to the hollow balloons of this expansive type, void balloons with holes are contained in a pre-expansion stage, and dusts, which have adhered to pipes and the like in the expansion process to be deposited thereon, and are burned through heat to form hard foreign matters, are also contained therein. Such hardened foreign matters remaining in urethane tend to cause scratches in the polishing process, resulting in a serious problem. In this manner, a producing process of a polyurethane foam by the use of hollow balloons tends to cause adverse effects.
The present invention has been devised to solve the above-mentioned problems, and its objective is to provide a producing method of a polishing pad-use polyurethane foam, which can produce a polyurethane foam that is evenly formed and less susceptible to deviations in specific gravity through processes without using hollow balloons.